A glass product is usually produced by melting glass raw materials by means of a glass melting furnace to form a glass melt and then forming the glass melt into a prescribed shape, followed by solidification. However, in order to obtain a homogeneous glass melt by means of a glass melting furnace, it was required to maintain the melting state over a very long period of time, whereby a large energy consumption was unavoidable.
In order to solve such a problem, a process for producing a glass product employing a technique so-called an in-flight melting method, has been proposed in which particles (granules) made of a mixture of glass raw materials are heated and melted in a gas phase atmosphere to form molten glass particles, and then, the molten glass particles are collected to form a liquid phase (glass melt) (e.g. Patent Document 1 and Non-Patent Document 1).
In a case where a glass product is produced by means of an in-flight melting method, usually, by a method of pneumatically conveying granules made of a mixture of glass raw materials, granules are supplied to an in-flight heating apparatus to melt the granules.
Further, it is known that granules to be used in the in-flight melting method can be produced by means of e.g. a spray drying granulation method (a spray drying method) (e.g. Patent Document 1).
Further, as a method for forming a mixed granulation glass raw material to be used for producing a glass product, a granulation method such as an agitation granulation method, a fluidized-bed granulation method, a tumbling granulation method, an extrusion granulation method or a dry granulation method is known (e.g. Patent Document 2).
Further, in a case where alkali-free glass is produced by a conventional melting method, not an in-flight melting method, a method is known wherein boric acid is incorporated in a glass material (material batch), water or a polyhydric alcohol is added, and granulation is carried out to improve the strength of granules (e.g. Patent Document 3).